Technical Field
The present disclosure relates to high dynamic range imaging.
Description of the Related Art
An image sensor conventionally comprises an array of pixels, each comprising a photodiode and a control circuit capable of delivering an output signal representative of the illumination level received by its photodiode. Such a sensor enables to acquire a discretized and digitized image of a scene (or digital image). Such a sensor can however only discriminate a finite number of levels of illumination received by each photodiode. It is thus not always possible to capture the entire illumination range or dynamic range available in a scene with a single time of exposure of the photodiodes, especially when the scene has a high contrast.
To obtain a high dynamic range image, it has already been provided to successively acquire a plurality of images of a same scene with different times of sensor exposure, and then to merge these images in a high dynamic range image, where the values of the image points or pixels corresponding to the darkest areas of the scene are based on the values of the corresponding pixels of the image acquired with the longest exposure times, and the values of the pixels corresponding to the brightest areas in the scene are based on the values of the corresponding pixels of the images acquired with the shortest exposure times.
A problem which arises is that certain scenes comprise blinking light sources, for example, lightings or street lights with pulsed light-emitting diodes. Such light sources turn on and off at relatively high frequencies, typically in the range from 50 Hz to 2 kHz, with lighting duty cycles (ratio of the on period to the off period) which may be relatively low, for example, in the order of 0.1. In many situations, there is a risk for the light emitted by such sources not to be detected or to be incorrectly detected by the sensor. This problem particularly arises in the field of image sensors embarked in motor vehicles to analyze the environment of the vehicles. Indeed, scenes acquired by such sensors often have a high contrast and may comprise blinking light sources.
FIG. 1 schematically illustrates this issue. FIG. 1 shows two successive phases Tframe of acquisition of a value representative of the illumination level of a photodiode of a pixel in a high dynamic range image sensor. Each acquisition phase Tframe comprises one or a plurality of periods of integration of the pixel photodiode. In the shown example, each acquisition phase Tframe comprises three successive periods TL, TM and TS of integration of the pixel photodiode. Time period TL is longer than time period TM, which is itself longer than time period TS. The duration of integration periods TL, TM and TS may vary from one acquisition phase to another according to the ambient brightness conditions, it being understood that the sum of times TL, TM and TS is always smaller than or equal to time Tframe, which is generally constant. At the end of the first integration period (period TL in this example), a first value representative of the illumination level of the photodiode is read and stored, and the photodiode is reset before the beginning of the second integration period (period TM in this example). At the end of the second integration period, a second value representative of the illumination level of the photodiode is read and stored, and the photodiode is reset before the beginning of the third integration period (period TS in this example). At the end of the third integration period, a third value representative of the illumination level of the photodiode is read. A final output value of the pixel is then determined by taking into account the three previously-acquired illumination level values, corresponding to integration time periods TL, TM and TS.
FIG. 1 further shows a binary signal LED representative of the state of a blinking light source placed opposite the pixel, for example, a pulsed light-emitting diode source. The high state of signal LED corresponds to an on state of the light source, and the low state of signal LED corresponds to an off state of the light source. In this example, the blinking frequency of the light source is substantially equal to the frequency of image acquisition by the sensor (equal to 1/Tframe), and the lighting duty cycle of the light source is 0.1. In the shown example, the light source is on for part of integration period TL, and is off during all the rest of phase Tframe and particularly during integration periods TM and TS. Thus, during an acquisition phase Tframe, the light emitted by the light source is measured by the photodiode only during long integration period TL of the photodiode. However, in many situations, the pixel photodiode saturates during long integration period TL of acquisition phase Tframe, and the value read at the end of long integration period TL then does not contribute to the final output value of the pixel. In this case, the turning-on of the light source during acquisition phase Tframe is not detected by the pixel.
It should be noted that for blinking sources having a blinking frequency of the same order of magnitude as the acquisition frequency of the sensor, and a short on-state time period as compared with acquisition period Tframe of the sensor, the probability of being in the situation shown in FIG. 1 is relatively high, since integration period TL generally occupies the most part of acquisition period Tframe.
When the ambient luminosity level increases, time periods TL, TM, and TS may be decreased and there then is a significant risk for the on period of the light source to fall outside of integration periods TL, TM, and TS of the sensor. Here again, the consequence is that the final output value of the pixel does not enable to detect the on state of the light source.
Further, when the ambient luminosity level is relatively high, there is a significant probability for the pixel to saturate during integration periods TL and TM (periods TL and TM cannot be decreased to keep a significant general dynamic range in the image). Only exposure period TS could then capture a non-saturated value. However, period TS then being very short, the probability of detecting the light of the blinking source is low.
There thus is a need for a high dynamic range image sensor capable of detecting blinking light sources with a greater reliability than existing sensors.